This invention pertains to housings for radios and more particularly to a thin, portable radio housing with an integral antenna.
An ultra thin electronic apparatus housing may be described as a housing having a thickness approximately equal to the thickness of several plastic credit cards, more or less. (A typical plastic credit card is slightly less than one millimeter thick.) These housings frequently have a length and width similar to a plastic credit card and, consequently, are often referred to as "credit card housings". The primary application for ultra thin housings has been in the pocket calculator art and U.S. Pat. No. 4,558,427 to Takeuchi et al. Describes and illustrates a housing suitable for a "credit card calculator".
There are other applications for ultra thin housings; for example, it would be desirable to have a selective call radio paging receiver (commonly called a "pager") contained within an ultra thin housing. Other radio receivers have already been integrated into ultra thin housings. In particular, the Casio Corporation, the assignee of the Takeuchi et al. patent, has manufactured a "credit card FM radio" (model RD-10) that receives commercial radio FM broadcasts. To hear a broadcast, the user wears a small earphone which is connected to the RD-10 by a cable. The earphone cable also functions as an antenna. Unfortunately, selective call radio paging receivers are not usually equipped with earphones. Thus, if a selective call paging receiver is to be integrated into an ultra thin housing ("a credit card pager") an integral antenna is necessary.
Because of its small size, it is anticipated that a credit card pager would be frequently carried very close to the body of the user; for example, in a shirt pocket. This presents a problem for an antenna that is positioned either on or within the ultra thin housing. Since the body is a good conductor, the electric field component of a received radio transmission is substantially shorted out near the surface of the body. This appears to eliminate electric field antennas, such as dipole antennas, as a suitable choice for a credit card pager.
The magnetic field component of a received radio transmission, however, is not shorted out close to the body and the lines of magnetic flux run parallel to the surface of the body. This seems to indicate that magnetic field antennas would be suitable for credit card pagers. However, there are two requirements that conflict when one attempts to position a magnetic loop antenna within an ultra thin housing; specifically, the cross-sectional area of a magnetic loop antenna should be as large as possible (more specifically, when the size of the loop antenna is less than half a wavelength) and the cross-sectional area of the loop (i.e., the "plane" of the loop) should be positioned perpendicular to the lines of magnetic flux. Since there are dramatic differences in the three dimensions of an ultra thin housing (length, width and depth; depth being extremely small) the antenna should be positioned parallel to the base and cover (i.e., parallel to length and width) to achieve maximum area. But this positions the loop parallel to the lines of magnetic flux, not perpendicular as required. If the loop is positioned perpendicular to the base or cover (i.e., perpendicular to length and width) the cross sectional area of the loop is very small. Thus, upon further analysis, it appears that a magnetic field loop antenna is also unsuitable for a credit card pager.
Experiments performed by the applicants, however, have indicated that a loop antenna positioned parallel to the base or cover of an ultra thin housing yields satisfactory performance when the housing is positioned close to the body. Applicants theorize that because the body is slightly curved and the housing is flat, the base of the housing can only be positioned parallel to the body at one point. Thus, the loop antenna is not positioned perfectly parallel to the lines of the magnetic flux, but at an acute, non-zero angle which permits the antenna to function.